Piezoelectric film has many advantages in the applications of transducers, sensors and electroacoustic devices. Piezoelectric polymeric materials include PVDF or PVF.sub.2 polyvinylidene fluoride), and its copolymer PVDF-TrFE (polyvinylidene fluoride--trifluoroethylene). When PVDF film is curved along the unidirectionally stretched direction and driven by an electrical field such as an AC field, displacement in the length direction is converted into the direction normal to the surface, and acoustic radiation results. This operates to generate an acoustic wave in the air. Using this design, piezoelectric material such as PVDF film can be used to make a loudspeaker or a microphone when the frequency of the generated acoustic wave is below 20 kHz. When the frequency is above 20 kHz, an ultrasound transducer may be formed.
A flat transducer of PVDF film material can also be made. U.S. Pat. No. 6,011,855, issued Jan. 4, 2000 to Selfridge et al., for example, discloses a speaker device for emitting subsonic, sonic or ultrasonic compression waves. The device includes a hollow drum, a rigid emitter plate attached to the drum, and a plurality of apertures formed within the plate which are covered by a thin flat layer of piezoelectric film disposed across the emitter plate. By using a negative or positive biasing pressure, a back cavity to create tension in the PVDF film, the device vibrates in substantially uniform fashion. However, such a structure and method has significant limitations, including requiring constant application of pressure differential in order to deform the PVDF material and to operate the transducer.
The beam angle of an ultrasonic transducer is controlled by the dimensions of the active area and the drive frequency of the transducer. The resonance frequency of a curved PVDF film (such as PVDF film curved into a partial cylinder, cylinder or sphere) depends on the curvature as well as the material properties of the PVDF film. For the same film, the frequency is higher when the radius of the curvature is small, and lower when the radius is larger. For a loudspeaker resonant at 1 kHz, the radius is about 20 cm. For an ultrasound transducer of 40 kHz, the radius is about 4-5 mm. Thus, it is important to design and control the curvature radius.
Curved PVDF may be used for a variety of applications, such as audio speakers and microphones and for ultrasound transducers with frequencies up to 180 kHz. For the application of omnidirectional ultrasound transducers, the resonance frequency of conventional transducers is limited by the radius/diameter of the film cylinder. For example, as depicted in FIG. 4D, transducer 90 having a frequency f.sub.1 of 40 kHz has a cylinder diameter D1 of about 1 cm. With a height H1 of about 1 cm, the active radiating area is about 3 cm.sup.2. As illustrated in FIG. 4E, for a transducer 90' with a frequency f.sub.2 of 100 kHz or higher, the diameter D2 of the cylinder would be significantly smaller than D1 (approximately 0.4 cm) and the radiating area would be smaller than 1.2 cm.sup.2. However, it is extremely difficult to fabricate such small PVDF film cylinders and ultrasound pressures resulting from their operation are similarly diminished due to the small radiating area.
Moreover, for a partial cylinder or sphere, the PVDF film needs to be supported by an annular/linear supporter in order to obtain the curved shape. In order to efficiently transmit an acoustic wave into the air, the edges of the film need to be rigidly clamped. Such ultrasonic transducers and methods of manufacture are illustrated in commonly assigned co-pending U.S. patent application Ser. No. 09/281,247 filed on Mar. 30, 1999 entitled "Omni-directional Ultrasonic Transducer Apparatus and Staking Method" and Ser. No. 09/281,398 filed on Mar. 30, 1999 entitled "Omni-directional Ultrasonic Transducer Apparatus Having Controlled Frequency Response", the contents of which are incorporated herein by reference. The requirement of the support to the film and clamping of the edges adds more complexity to the design and manufacturing cost of these types of transducers. For ultrasound transducers, this problem becomes more severe due to the small radius of the curvature and small dimension of the transducer.
A transducer which overcomes the above problems, including frequency limiting on the whole cylinder design and mechanical clamping in the partial cylinder designs, as well as a method of making such a curved PVDF film airborne transducer which has a resonance frequency which is not influenced by thickness, cylinder diameter, or shape is highly desired.